1. Field of the Disclosure
The disclosure relates generally to waveguide lasers and, more particularly, to waveguide lasers with a fiber gain medium.
2. Brief Description of Related Technology
Fiber lasers are often the optical sources of choice for demanding environments and stringent field-deployment requirements. Fiber lasers are generally regarded as possessing a number of advantageous characteristics. They are considered quite versatile, with operation at various wavelengths and the capability of being tuned over large wavelength ranges. Fiber lasers have been designed to generate short or ultrashort pulses, or have been used to generate very narrow linewidth pulses. They can be efficiently diode-pumped. They have a very high surface to volume ratio, which provides for excellent cooling of the lasing medium. Without such cooling, thermal loading (i.e., excessive heat buildup) can limit laser performance, especially in field usage.
Fiber lasers are also generally considered field-compatible because the “intracavity” light propagation is tightly guided by the fiber. The fiber medium guides the optical signal and protects it from environmental perturbations.
Unfortunately, the insensitivity of fiber lasers to the environment is generally not achieved in reality. The versatility and advantageous characteristics of fiber lasers often comes at the cost of environmental sensitivity. For example, while the frequency of some fiber lasers can be tuned, such tuning typically requires free-space components that compromise the robustness and the stability of the fiber laser. For more advanced fiber lasers, this environmental sensitivity is a serious problem.
Tuning is not the only feature generally associated with advanced fiber lasers that can be achieved only with awkward add-on components that diminish the robustness of the laser. Efforts to apply fiber lasers in scenarios involving both tuning and short pulse generation have also relied on such arrangements. See, for example, C. Erny, et al., “Mid-infrared difference-frequency generation of ultrashort pulses tunable between 3.2 and 4.8 μm from a compact fiber source,” Opt. Lett., Vol. 32, pp. 1138-1140 (2007). Because fiber lasers are composed of fiber-based and non-fiber-based elements, the light signal, as it propagates through the laser cavity, must exit and re-enter the fiber section. While exiting the fiber is generally relatively simple (at least for low power/low energy system), re-entering the fiber after traveling through an air gap is quite a difficult task. The optical signal generally must be pointed onto the fiber core. The fiber core is typically 6-8 microns in diameter (e.g., for single-mode fibers). As a result, even minute mechanical drifts will strongly affect the coupling of the optical signal into the fiber. Furthermore, any dust or other contaminants that land on the fiber core will block the incoming signal either partially or fully. In some cases, a contaminant can burn onto the face of the fiber, causing permanent damage. Moreover, the air gap also introduces a segment where the beam propagates in an open environment where it is subject to air currents.
Past attempts at addressing the problems of in-fiber and out-of-fiber transitions have tried to lessen this problem by developing special ends of fiber elements, e.g., coreless fiber segments, sapphire fiber tips, etc. While these approaches may help lessening some of the in-fiber and out-of-fiber transitions issues, they introduce new problems by, for instance, being complex and often very bulky.
For the foregoing reasons, advanced fiber lasers are not made simply out of fibers and fiber-based components. Fiber lasers instead contain numerous free-space components (i.e. components where the light propagates unconstrained by a physical guide). See, for example, the traditional fiber laser described in U.S. Pat. No. 5,627,848. Free space components and the foregoing fiber transitions significantly degrade performance and introduce difficulties in demanding field-applications. Thus, fiber lasers have, in theory, numerous desirable features as a source for operation in demanding environments. Nonetheless, in practice, it is generally not possible to fully exploit these features, because all but the simplest fiber lasers contain extraneous components where the light propagation is unconstrained by waveguiding elements and exposed to the environment.